Valve actuation in an internal combustion engine is required in order for the engine to operate. During the intake portion of the cycle, one or more intake valves may be opened to admit air or fuel and air into a cylinder for combustion. Subsequently, during the exhaust portion of the cycle, one or more exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake, exhaust, and/or auxiliary valves also may be opened at various times to provide engine braking and to recirculate gases from the intake and exhaust manifolds to engine cylinders.
Engine valve actuation may be used to produce engine braking and exhaust gas recirculation (EGR) when the engine is not being used to produce power. During engine braking, the exhaust valves may be selectively opened to temporarily convert the engine into an air compressor. In doing so, the engine develops retarding torque to help slow the vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle.
In many internal combustion engines, the intake and exhaust valves may be opened and closed by fixed profile cams, and more specifically by one or more fixed lobes that are an integral part of each of the cams. Benefits such as increased performance, improved fuel economy, lower emissions, and better vehicle drivability may be obtained if the intake and exhaust valve timing and lift can be varied. The use of fixed profile cams, directly opening and closing the valves, however, can make it impractical to adjust the valve timings and lift.
One proposed method of adjusting valve timing and lift, given a fixed cam profile, has been to provide a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is a technique for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage assembly. In a lost motion system, a cam lobe may provide the “maximum” (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage used to actuate the valve to be opened, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a reduced amount of the cam motion to the valve. Lost motion systems which are used to provide variable valve actuation on an engine cycle-by-cycle basis must be sufficiently fast to allow the variable length link to vary within the duration of the cam rotational period. An example of such a system and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application.
Engine benefits from lost motion variable valve actuation systems can be achieved by creating complex cam profiles with extra lobes or bumps to provide auxiliary valve lifts in addition to the conventional main intake and exhaust events. For example, an intake cam profile may include an additional lobe for exhaust gas recirculation (EGR) prior to the main intake lobe, and/or an exhaust cam profile may include an additional lobe for EGR after the main exhaust lobe. Other auxiliary lobes for any number of engine valve events, such as cylinder charging and/or compression release braking may also be included on the cams. The lost motion variable valve actuation system may be used to selectively cancel or activate any or all combinations of valve lifts possible from the assortment of lobes provided on the intake and exhaust cams. As a result, significant improvements may be made to both power and engine braking operation of the engine.
In one lost motion valve activation system, the displacement applied to the valve is a combination of displacements created by a control piston and a valve seating device as well as the cam. One possible implementation of lost motion valve actuation employs a finger follower assembly to combine these displacements and apply the result to the valve. Preferred finger followers may have low mass and provide low inertia. It is therefore an advantage of some, but not necessarily all, embodiments of the present invention to provide the foregoing capabilities. Additional advantages of various embodiments of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.